The week of the January 2008 stock market downturn was characterised by extreme volatility in the markets, and saw both the biggest one day percentage fall and rise in the IBEX 35's history. The index closed 7.5% down on 21 January2008, the second biggest fall in the Spanish equity market since 1987, and rose a record 6.95% three days later.

Rules

Selection

The composition of the IBEX 35 is reviewed twice per year (in June and December) by the so-called Technical Advisory Committee, which consists of "representatives of the stock exchanges and derivatives markets, as well as... renowned experts from the academic and financial fields". If any changes are made, they are
effected on the first trading day of July or January. In general, at each review, the 35 companies with the highest trading volume in Euros over the previous six months are chosen for inclusion in the index, provided that the average free floatmarket cap of the stock is at least 0.3% of the total market cap of the index. Any candidate stock must also have either been traded on at least a third of all trading days in the previous six months, or rank in the top twenty overall in market cap (thus allowing large recently IPO'ed companies to be included).

Weighting

The IBEX 35 is a market value-weighted index. The market cap used to calculate the weighting of each constituent is multiplied by a free float factor (ranging from 0.1 to 1) depending on the fraction of shares not subject to block ownership. Any company with 50% or more of its shares considered free float is given a free float factor of 1. Unlike many other European benchmark indices, the weightings of companies in the IBEX 35 are not capped.

Calculation

The index value (given here as I) of the IBEX 35 index is calculated using the following formula:

with t the moment of calculation; Cap the free float market cap of a specific listing and J a coefficient used to adjust the index on the back of capital increases or other corporate actions so as to ensure continuity. The formula can be adjusted to accommodate changes in index structure, such as the temporary suspension of companies pending news.